Thermal and Mechanical Properties of Polystyrene Modified with Esters

J Therm Anal Calorim (2015) 121:235–243 DOI 10.1007/s10973-015-4547-7

Thermal and mechanical properties of polystyrene modiﬁed with esters derivatives of 3-phenylprop-2-en-1-ol

Marta Worzakowska

Received: 14 September 2014 / Accepted: 6 February 2015 / Published online: 3 March 2015 Ó The Author(s) 2015. This article is published with open access at Springerlink.com

Abstract The thermal and mechanical properties of utilized as plastics, latex paints, coating, synthetic rubbers polystyrene (PS) modified with esters derivatives of and styrene alkyd coatings, for food-contact packing 3-phenylprop-2-en-1-ol were investigated. The influence of polymers, in electronics and building materials, as material the content of esters on the glass transition temperature, for the formation of toys, cups, office supplies, etc. [1–3]. dynamic mechanical properties, flexural properties, hard- On the contrary, styrene easily copolymerizes with differ- ness and thermal stability of PS has been examined. It was ent monomers such as acrylonitrile, methacrylamide, di- found that the PS/ester compositions were characterized by vinylbenzene, butadiene, maleic anhydride, vinyl chloride, lower stiffness, lower values of Tg, lower hardness, lower esters of organic acids, e.g., acrylates or methacrylates, stress at break, lower thermal stability and higher values of unsaturated polyesters or others creating polymeric mate- tg delta height and strain at break as compared to pure PS. rials with unique properties suitable for many industrial The obtained results proved that esters derivatives of applications [4–14]. 3-phenylprop-2-en-1-ol can find their place as an envi- In order to improve the processing, performance and ronmentally friendly, external plasticizers of PS. elasticity of plastic materials, the polar and non-polar addi- tives (plasticizers) are added. The interactions of plasticizer Keywords Polystyrene Á Thermal properties Á molecules with polymer chains cause disruption of the sec- Viscoelastic properties Á Flexural properties Á Hardness Á ondary valence bonds or van der Waals force between Plasticizers polymer molecules. As a consequence, a decrease in the intermolecular interactions and thus an increase the mobility of the polymer chains are observed. As a result, the materials Introduction are characterized by lower moduli, stiffness, glass transition temperature and hardness. Meanwhile, the ability of mate- Polystyrene (PS) is considered to be the most durable rials for elongation and polymer chain flexibility sig- thermoplastic polymer. It is used in a wide range of nificantly increase [15–17]. The most, generally applied products due to its versatile properties. Polystyrene is plasticizers are low molecular mass organic compounds, characterized by the resistance to biodegradation, stiffness which are characterized by low volatility in order to prevent or flexibility (with plasticizers), light weight, good optical, their rapid evaporation from manufactured products. Among chemical and insulation properties and facile synthesis. It is commercially applied plasticizers for PS, phthalate esters such as dimethyl, diethyl, dipropyl, dibutyl, diheptyl, dioc- tyl, diisodecyl or benzylbutyl phthalates are the most com- monly used [18–21]. In addition, the application of adipate and glutarate esters as plasticizers for the expanded PS and M. Worzakowska (&) the liquid paraffin and zinc stearate as an internal plasticizers Department of Polymer Chemistry, Faculty of Chemistry, Maria is reported [22, 23]. However, most of phthalates have toxic Curie-Skłodowska University, Gliniana 33 Street, 20-614 Lublin, Poland properties for human. Due to this, the intensive studies on the e-mail: [email protected] new, non-toxic and biodegradable materials that could 123 236 M. Worzakowska replace harmful plasticizers are developed [24, 25]. In recent Experimental years, the utilization of eco-friendly plasticizers such as epoxidized vegetable oils, biodiesel oils, hydrogenated Materials Castrol oil, citrate esters, poly(ethylene glycol) of low molecular weight or core-hydrogenated phthalates has been Esters derivatives of 3-phenylprop-2-en-1-ol were prepared investigated [26–30]. through catalyzed esterification process of 3-phenylprop-2- The main objective of this paper is to study the en-1-ol (98 %, Fluka) and acidic reagents such as succinic thermal and mechanical properties of PS modified with anhydride (99 %, Merck) or sebacic acid (98 %, Merck) esters derivatives of 3-phenylprop-2-en-1-ol. This alcohol according to the method described in Ref. [32]. The occurs in nature in many oils and balsams such as cassia, structure of esters is shown in Scheme 1. The following styrax, hyacinth oils or Peru and Honduras balsams [31]. abbreviations for esters were used as follows: CBE (ester The esters of 3-phenylprop-2-en-1-ol are aromatic- of 3-phenylprop-2-en-1-ol and succinic anhydride) and aliphatic compounds, which differ in their structure and CSE (ester of 3-phenylprop-2-en-1-ol and sebacic acid). thus in their properties. The ester of 3-phenylprop-2-en- Polystyrene was obtained by free-radical polymerization of 1-ol and succinic anhydride (CBE) contains two methy- styrene (POCh, Gliwice, Poland) in the presence of benzoyl lene groups (–CH2–), but the ester of 3-phenylprop-2-en- peroxide (1.0 mass%) as an initiator (POCh Gliwice, 1-ol and sebacic acid (CSE) contains eight methylene Poland). The reaction was carried out at 60 °C. Raw PS groups (–CH2–) in their chain spacer. CBE is a liquid was washed with methanol in order to remove un-reacted with boiling temperature of 210 °C; however, CSE is a monomer and benzoyl peroxide. After filtration, bulk PS solid with melting and boiling temperatures of 92 and was dried to a constant mass. The obtained bulk PS was 260 °C, respectively. It is worth noting that those esters characterized by SEC method. The average molecular mass have high thermal stability and thus low volatility. The and a polydispersity of prepared, bulk PS were 105, 000 thermal decomposition of CBE starts about 220 °C. and 2.7, respectively. However, the beginning of the thermal decomposition of CSE is visible at 270 °C. CBE and CSE are slowly Sample preparation hydrolyzable, well-soluble compounds in organic sol- vents and well miscible with thermoplastic polymers The PS/ester compositions were prepared by solution [32]. Due to their properties, they can find their place as blending. Polystyrene was dissolved in hot chloroform, and potential, eco-friendly plasticizers for specific applica- then suitable amounts of esters were added. The solutions tions, especially in the areas where humans have a direct were precisely mixed. The obtained blends were deposited contact, e.g., for the production of toys, medical products and spread over glass plate. The samples were kept for and food packing. In order to check their action on the 5 days at room temperature and then at 60 °C under thermal and mechanical properties of chosen, commer- vacuum for 2 days in order to evaporate the solvent. The cially used thermoplastic polymer such as bulk PS, the compositions contain from 0.5 to 20 mass% of esters were compositions containing different ester content are pre- prepared. In addition, samples of pure PS were also pared. PS and esters were mixed together making the manufactured to compare the results. compositions containing from 0.5 to 20 mass% of ester. The influence of the content of esters and the structure of esters on the glass transition temperature, storage Methods modulus, Young modulus, stress and strain at break, hardness and thermal stability of prepared materials has Differential scanning calorimetry analysis (DSC) was car- been evaluated and discussed. ried out with a DSC 204 calorimeter, Netzsch (Germany).

Scheme 1 Structure of esters, O O where CBE is ester of 3-phenylprop-2-en-1-ol and C CH CH2 OCCH2CH2 C OCH2 C C succinic anhydride, CSE is ester H H H of 3-phenylprop-2-en-1-ol and CBE sebacic acid O O

C CH CH2 OCCH2CH2CH2CH2CH2CH2CH2CH2 C OCH2 C C H H H

CSE

123 Thermal and mechanical properties of polystyrene 237

All DSC measurements were carried out in aluminum pans Table 1 DSC data of PS/CBE compositions with pierced lid. As a reference empty aluminum crucible CBE content/% Tg/°C Tmax1/°C Tmax2/°C was applied. The mass of the sample was about 10 mg. The dynamic scans were performed at a heating rate of 0 96 – 425 10 °C min-1 from 20 to 500 °C under argon atmosphere 0.5 96 – 423 (20 mL min-1). 1 96 234 421 Dynamic mechanical analysis (DMA) was performed on 3 92 230 419 a DMA Q 800 TA Instruments (USA). Tests were con- 5 90 242 420 ducted with a double Cantilever device with a support span 10 89 233 417 of 35 mm. Measurements for all samples were made from 20 65 256 416 room temperature up to temperature until the sample become too soft to be tested. A constant heating rate of 6 °C min-1 and an oscillation frequency of 10 Hz were applied. The rectangular proﬁles of 10-mm-wide and Table 2 DSC data of PS/CSE compositions

2-mm-thick samples were used. The storage modulus CSE content/% Tg/°C Tmax1/°C Tmax2/°C 00 (E20 °C, E30 °C), loss modulus (E ), tg delta maximum and tg delta height were evaluated. 0 96 – 425 Tensile properties were determined using a Zwick Roell 0.5 95 – 420 Z010 testing machine (Germany). The specimen dimen- 1 94 230 418 sions were 10 mm wide and 2 mm thick. The measure- 3 90 231 415 ments were carried out at room temperature with the 5 92 234 414 crosshead speed of 2 mm min-1. Young modulus, stress at 10 87 238 413 break and strain at break were determined. 20 60 240 410 Hardness according to Brinell (HK) was evaluated by means of a hardness tester HPK and calculated based on following equation: HK [MPa] = F1 * 0.098066, where F1 is a strength of pressure under deﬁnite load. 2000 — 0 % Thermal analysis (TG) was carried out on an STA 449 —0.5 % 1600 —1% Jupiter F1, Netzsch (Germany) equipped with a sensor —3% thermocouple-type S TG-DSC. All measurements were —5% 1200 —10 % made in Al2O3 crucibles. As a reference empty Al2O3 - - 20 % crucible was applied. Dynamic scans were performed at a heating rate of 10 °C min-1 from 40 to 700 °C under he- 800 lium and air atmospheres (25 mL min-1). The sample mass was about 10 mg. Storage modulus/MPa 400 The gas composition evolved during heating of studied materials was detected and analyzed by a FTIR spec- 0 20 40 60 80 100 120 140 160 trometer Brucker TGA 585 (Germany) coupled on-line to Temperature/°C STA instrument. The FTIR spectra were collected in the 2 spectral range from 600 to 4000 cm-1 with a resolution of -1 4cm and 16 scans per spectrum. 1.6

1.2 Results and discussion

tg delta 0.8 The DSC data are placed in Tables 1 and 2. Based on the presented results, it is clearly visible that the glass transi- 0.4 tion temperature (Tg) for pure PS is 96 °C and it is in agreement with the reported values [33, 34]. It can be also 0 observed that with increasing the amount of added ester 20 40 60 80 100 120 140 160 Temperature/°C from 0.5 to 10 mass%, the Tg values of obtained materials slightly decrease. However, when the amount of esters is Fig. 1 Storage modulus and tg delta versus temperature for PS and higher than 10 %, the changes in Tg values become more PS/CBE compositions 123 238 M. Worzakowska

signiﬁcant. The Tg values are 65 °C for the PS/20 mass% Tg values between the PS/lower chain-length ester (CBE) CBE and 60 °C for the PS/20 mass% CSE. In addition, as compositions and the PS/higher chain-length ester (CSE) can be seen from Tables 1 and 2, only small differences in compositions are observed. Generally, DSC curves of all

Table 3 DMA data and HK values of PS/CBE compositions

0 0 00 CBE content/% E 20°C/MPa E 30°C/MPa tg delta/°C E /°C tg delta HK/MPa

0 1825 1780 132 99 1.05 143 0.5 1760 1730 123 99 1.32 130 1 1620 1560 106 88 1.42 130 3 1550 1510 104 88 1.62 129 5 1440 1320 104 85 1.73 128 10 1440 1320 102 81 1.88 123 20 1225 1090 77 61 1.94 98 Where HK hardness according to Brinell

Table 4 DMA data and HK values of PS/CSE compositions

0 0 00 CSE content/% E 20°C/MPa E 30°C/MPa tg delta/°C E /°C tg delta HK/MPa

0 1825 1780 132 99 1.05 143 0.5 1650 1630 118 99 1.42 130 1 1520 1430 98 85 1.53 128 3 1435 1390 97 85 1.77 127 5 1310 1265 96 83 1.85 127 10 1280 1210 95 78 1.93 120 20 1050 1005 65 57 2.15 85 Where HK hardness according to Brinell

Table 5 Mechanical properties of PS/CBE compositions CBE content/% Young modulus/MPa Stress at break/MPa Strain at break/%

0 1950 35 3.2 0.5 1870 35 3.2 1 1800 33 3.3 3 1740 30 3.6 5 1650 28 3.6 10 1590 25 4.0 20 1350 20 5.9

Table 6 Mechanical properties of PS/CSE compositions CSE content/% Young modulus/MPa Stress at break/MPa Strain at break/%

0 1950 35 3.2 0.5 1890 33 3.3 1 1850 30 3.5 3 1770 30 3.7 5 1680 26 3.9 10 1620 23 4.3 20 1390 18 6.5

123 Thermal and mechanical properties of polystyrene 239

110 studied materials show one, asymmetric, non-well 0 % separated endothermic signal with one or two maxima 90 0.5 % 1 % (Tmax1 and Tmax2), which is directly connected with the 3 % decomposition of the studied materials [35–37]. In addi- 70 5 % tion, the presented data suggest that the characteristic de- 10 % 50 20 % composition temperatures are almost independent on the TG/% content of esters in the PS compositions. 30 In Fig. 1, the storage modulus and tg delta in the func- tion of temperature for the obtained materials are pre- 10 sented. It can be seen that the major changes in storage modulus values when the materials pass through glassy –10 40 140 240 340 440 540 640 state to rubbery state are observed. The storage modulus of Temperature/°C pure PS appointed at 20 °C(E20 °C) is 1825 MPa. How- 5 ever, the storage modulus for the PS/esters compositions ranges from 1050 up to 1760 MPa, as shown in Tables 3 0 and 4. It means that PS/ester compositions are character-

–5 ized by lower stiffness as compared to pure PS. In addition, –1 the same trend is observed for glass transition temperature –10 (marked from DSC and DMA analyses) of prepared materials. The glass transition temperature (T )(a relaxation) DTG % min % DTG g –15 was qualiﬁed as the maximum of loss modulus (onset glass

–20 transition temperature) and as the maximum of peak of tg delta (midpoint glass transition temperature) [38]. DMA –25 plots show that Tg values decrease with the increase in the 40 140 240 340 440 540 640 Temperature/°C amount of ester in the compositions. The same trend was observed for Tg values marked from DSC curves. It is Fig. 2 TG and DTG curves of PS and PS/CBE compositions in inert worth noting that Tg values appointed as the maximum of atmosphere

Table 7 TG and DTG data of PS/CBE compositions in inert atmosphere

CBE content/% First mass loss/% IDT1/°C Tmax1/°C FDT1/°C Second mass loss/% Tmax2/°C FDT2/°C

0 – 325 – – 100 412 426 0.5 – 323 – – 100 410 426 1.0 7.6 230 231 250 92.4 413 429 3.0 7.9 240 237 251 92.1 409 427 5.0 8.2 242 235 245 91.8 410 429 10.0 11.2 250 242 255 88.8 405 428 20.0 19.8 203 272 287 84.2 401 423 IDT initial decomposition temperature (expressed as the temperature where 5 % of mass loss is observed); FDT ﬁnal decomposition temperature

Table 8 TG and DTG data of PS/CSE compositions in inert atmosphere

CSE content/% First mass loss/% IDT1/°C Tmax1/°C FDT1/°C Second mass loss/% Tmax2/°C FDT2/°C

0 – 325 – – 100 412 426 0.5 – 320 – – 100 412 426 1.0 7.0 230 235 255 93.0 410 430 3.0 7.5 243 239 257 92.5 407 432 5.0 8.0 245 235 243 92.0 406 430 10.0 10.7 225 236 248 89.3 403 425 20.0 20.5 200 245 267 83.2 400 420 IDT initial decomposition temperature (expressed as the temperature where 5 % of mass loss is observed); FDT ﬁnal decomposition temperature

123 240 M. Worzakowska

00 E are in accordance with Tg values evaluated based on a 110 DSC analysis. It can be observed from Fig. 1 that the tg 90 delta height that is attributed to the mobility of the resin — 0 % — 0.5 % molecules [39–42] increases as the content of ester is in- 70 creased in the compositions. It testiﬁes to higher elasticity — 1 % — 3 % of the PS/ester compositions than pure PS. In addition, the 50 — 5 %

TG/% — 10 % PS/CSE compositions exhibit lower values of storage - - 20 % 30

(a) 10

–10 40 140 240 340 440 540 640 Temperature/°C

0 Absorbance/a.u. –1 –4

600 1100 1600 2100 2600 3100 3600 DTG/% min –8 Wavenumber/cm–1

(b) –12 40 140 240 340 440 540 640 Temperature/°C

Fig. 4 TG and DTG curves of PS and PS/CBE compositions in oxidative atmosphere

Absorbance/a.u. modulus, glass transition temperature and higher values of tg delta height as compared to pure PS and the PS/CBE compositions. 600 1100 1600 2100 2600 3100 3600 As follows from the data in Tables 3 and 4, increasing Wavenumber/cm–1 the content of ester causes a gradual decrease in the hardness (HK) of the studied compositions. The hardness (c) of pure PS is equal to 143 MPa. However, the hardness of the PS/ester compositions is from 130 (0.5 mass%) to 120 MPa (10 mass%). The highest drop of hardness values is observed for the PS/20 mass% ester compositions (98 and 85 MPa, respectively). The tensile properties of studied materials are placed in Tables 5 and 6. The data indi- cate that the addition of esters to the PS has signiﬁcant

Absorbance/a.u. inﬂuence on the mechanical properties of prepared compositions. Generally, the PS/esters compositions are characterized by smaller Young modulus and tensile strengths values than pure PS. However, with the increase in the 600 1100 1600 2100 2600 3100 3600 amount of ester in the compositions, the strain at break –1 Wavenumber/cm ultimately increases as well. Fig. 3 FTIR spectra of gaseous products emitted during decompo- Figure 2 presents the TG/DTG curves of obtained ma- sition of PS a and PS/20 mass% of CBE gathered at Tmax1 b and Tmax2 terials in inert atmosphere. In addition, the TG/DTG data in c in inert atmosphere inert atmosphere are gathered in Tables 7 and 8. One can

123 Thermal and mechanical properties of polystyrene 241 see that pure PS decomposes in one main stage that is humidity from the atmosphere was highly expected. visible from 325 °C to almost 426 °C with Tmax 412 °C. However, one can assume that the second DTG peak ob- Those observations are in accordance with the literature served at Tmax2 is due to the decomposition of PS. Ac- data where Tmax of decomposition of PS is contained be- cording to literature survey, the main reaction is the tween 425 and 429 °C depending on the synthesis and breakage of C–C bonds in the main chain of PS under analysis conditions [40–44]. The addition of esters to PS pyrolysis. This step is directly connected with a free-radical leads to a displacement of the initial decomposition tem- chain reaction—depolymerization of PS. It leads to the perature of obtained materials toward lower temperatures. formation of styrene as a main decomposition product and Meanwhile, an increase in the number of the degradation also some amounts of toluene and ethylbenzene [45–48], as stages is observed together with the presence of esters. The shown in Fig. 3. PS/esters compositions decompose in two main stages. The The TG/DTG curves of the studied materials under air

first is appeared at lower temperatures (Tmax1). The mass conditions are presented in Fig. 4. The results show that pure loss is from 7.0 to 20.5 % depending on the ester structure PS decomposes in one main step under oxidative conditions. and ester content. This decomposition stage is directly However, pure PS is characterized by significant lower ini- connected with the presence of esters in the studied ma- tial decomposition temperature (IDT) and the temperature of terials. In this temperature range, mainly the emission of the maximum rate of mass loss (Tmax1) under air conditions, gaseous products formed during pyrolysis of esters is ex- in Table 9, than under inert conditions, in Table 7. On the pected. As it was already confirmed, the pyrolysis of CBE contrary, the decomposition of the PS/ester compositions and CSE leads to the production of CO2, CO and H2Oasa runs as two-stage processes. The first decomposition step is main gaseous products and small amount of other organic observed at Tmax1 370–395 °C. The mass loss in this step is decomposition products such as benzene, toluene, styrene, from 96 to 99.5 %. The second stage occurs at Tmax2,in ethylbenzene, aliphatic, saturated aldehydes or carboxylic Tables 9 and 10. According to the FTIR spectra gathered at acids [32]. The presented FTIR spectrum of the gaseous Tmax1 and Tmax2, Fig. 5, one can see that CO2 and H2O are products emitted at Tmax1 confirms this supposition, as the main decomposition products of pure PS and PS/ester shown in Fig. 3. In addition, the evaporation of humidity is compositions under air conditions in both stages. It indicates expected in this stage. The samples for the TG studies were on the thermooxidative degradation of the studied materials used in the form of powders and thus the absorption of the [32, 49, 50].

Table 9 TG and DTG data of PS/CBE compositions in oxidative atmosphere

CBE content/% First mass loss/% IDT1/°C Tmax1/°C FDT1/°C Second mass loss/% Tmax2/°C FDT2/°C

0 100 280 380 450 – – – 0.5 99.2 260 375 414 0.8 475 563 1.0 99.3 258 380 414 0.7 500 555 3.0 99.2 245 375 413 0.8 490 560 5.0 99.1 245 370 415 0.9 530 558 10.0 97.4 240 380 418 2.6 510 560 20.0 96.0 232 395 428 4.0 532 587 IDT initial decomposition temperature (express as the temperature where 5 % of mass loss is observed); FDT ﬁnal decomposition temperature

Table 10 TG and DTG data of PS/CSE compositions in oxidative atmosphere

CSE content/% First mass loss/% IDT1/°C Tmax1/°C FDT1/°C Second mass loss/% Tmax2/°C FDT2/°C

0 100 280 380 450 – – – 0.5 99.5 260 375 418 0.5 480 565 1.0 99.0 255 380 417 1.0 490 560 3.0 99.0 245 380 415 1.0 490 560 5.0 99.4 240 375 415 0.6 520 565 10.0 98.2 238 385 420 1.8 515 565 20.0 96.5 230 390 425 3.5 535 590 IDT initial decomposition temperature (express as the temperature where 5 % of mass loss is observed); FDT ﬁnal decomposition temperature

123 242 M. Worzakowska

temperature, storage modulus, Young modulus, stress at (a) break, hardness, thermal stability and higher values of tg delta height and strain at break as compared to pure PS. In addition, it was found that CSE had higher inﬂuence on the decreasing the intermolecular interactions and thus the increasing the mobility of the polymer chains of PS than CBE. The studies proved that esters derivatives of Absorbance/a.u. 3-phenylprop-2-en-1-ol can be utilized as external, envi- ronmentally friendly plasticizers for commercially used thermoplastic polymers such as PS. They can be suitable alternative to widely, industrially applied compounds such 600 1100 1600 2100 2600 3100 3600 as toxic phthalates. Wavenumber/cm–1 Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, dis- (b) tribution, and reproduction in any medium, provided the original author(s) and the source are credited.

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